1. Field of the Invention
The present invention relates to a circuit arrangement for frequency modulation.
2. Description of the Background Art
Circuit arrangements for frequency modulation usually include a voltage-controlled oscillator whose (resonant) frequency is modulated or changed as a function of a modulation signal. To this end, the voltage-controlled oscillator typically contains one or more so-called varactor or capacitance diodes, i.e., components whose capacitance changes as a function of a control voltage that is applied to them. As a result of the capacitance change, the resonant frequency of a resonant circuit that includes the varactor diode or diodes changes. Consequently, the voltage-controlled oscillator is an electrical oscillation source whose frequency can be modulated or changed by the application of a modulation signal. The varactor diode constitutes the frequency-determining element in this regard. Changing the voltage on the varactor diode changes its capacitance and thus the resonant frequency of the resonant circuit.
The capacitance vs. voltage characteristic curve of a varactor diode is not linear. Thus, when a modulation signal or a modulation voltage is used that deviates sinusoidally from a center value, for example, distortions arise as a result of the nonlinear capacitance vs. voltage characteristic curve. These distortions cause, for example, a frequency offset or a frequency shift of the frequency-modulated signal thus produced as compared to a case where a linear relationship exists between modulation voltage and capacitance. This frequency shift can cause errors in a receiver of the frequency-modulated signal, for example in evaluating the demodulated receive signal.